1. Technical Field
The present invention relates to a driving method for a liquid crystal display apparatus, a liquid crystal display apparatus, and an electronic device.
2. Related Art
Active matrix liquid crystal display apparatuses using liquid crystals are known. The driving methods for such past active matrix liquid crystal display apparatuses can be roughly divided into an analog driving method and a digital driving method.
With the analog driving method, analog voltages are applied to pixels within a frame, and tones are expressed through the orientation states of the liquid crystals resulting from the applied voltages. Meanwhile, frame-inversion driving system, in which the polarity inversion cycle is completed with each frame write, is generally used, with a cycle of 60 Hz or, with the recent double-speed intermediate frame technology, 120 Hz being the mainstream driving rates.
On the other hand, with the digital driving method, each frame in an image signal is configured of multiple subfields (SFs) shorter than a single frame period, and the display is driven by selectively controlling each subfield to turn on or off.
For example, JP-A-2005-352457 discloses a liquid crystal display apparatus in which the subfield period in each subfield is divided into a former half and a latter half, and the positive/negative polarity of the voltage applied to the liquid crystals is inverted thereby, thus achieving AC driving (inverted driving).
JP-A-2005-352457 submits that this driving method makes it possible to suppress the cancellation of DC components applied to the liquid crystals, which causes the image problem known as flicker, and to suppress degradation of the liquid crystal material caused by the application of DC voltage. The digital driving method has thus been able to accelerate the polarity inversion cycle beyond that of the analog driving method.
It is also known that in liquid crystal display apparatuses, impurity ions are produced within the panel due to manufacturing issues, temporal change of the liquid crystals, and so on. If the produced impurity ions are adsorbed onto the alignment layer (on the substrate side) or the like, display deficiencies such as drops in contrast, luminance variance due to differences in the distribution of the adsorbed impurity ions, and so on will arise.
To be more specific, depending on their polarities, the impurity ions are adsorbed onto sides of the substrate (alignment layer, electrode, or the like) based on differences in potential caused by the voltages applied to each pixel, and a reverse electric field relative to the applied voltages is formed by the adsorbed impurity ions. To rephrase, a reverse electric field of the direction that weakens the applied voltages is formed by the adsorbed impurity ions. The situations in which this reverse electric field occurs differ depending on the resistance of the liquid crystals as well, and while it is possible to suppress changes in the potential within the frame period by using high-resistance liquid crystals, changes in potential occur within the frame period in panels that have seen extended use, panels having low resistances due to their specifications, and so on, and thus the problem of display deficiencies occurring has persisted.
However, in the past analog driving method, the aforementioned polarity inversion is slow as being a single frame unit, and the influence of voltage asymmetry is great to affect the liquid crystal. This accelerates the liquid crystal degradation, leading in turn to increased production of impurity ions, which results in the problem of impurity ions adsorption.
On the other hand, with the digital driving method, the polarity inversion cycle is too fast, and thus the liquid crystal response involved with the polarity inversion cannot keep pace, which means that polarity inversion cannot be carried out to the fullest extent; this causes a problem in that it is difficult to suppress the adsorption of impurity ions. To be more specific, a liquid crystal response time of approximately 2 ms is generally considered to be fast, but JP-A-2005-352457 discloses a minimum SF period of 5 μs and a maximum SF period of 300 μs, and thus the liquid crystal response cannot keep pace even if the positive-negative polarity is reversed in half of such a cycle.
In other words, with the past driving methods, it has been difficult to suppress display deficiencies caused by impurity ions, and thus there has been demand for a driving method that is effective against such voltage asymmetry.